The investigation is concerned with the damaging effect of ultraviolet light on the nucleic acid of the bacterial cell to prevent DNA replication and interfere with RNA formation. It is devoted to studies of the cellular recovery mechanisms which result in resumption of normal macromolecular synthesis and the mistakes made in the process of these repair mechanisms which lead to mutation. It includes studies of the induction of non-photoreversible damage blocking normal semi- conservative DNA replication and presumably leading to a requirement for postirradiation RNA and protein formation in restoration of a new origin for DNA replication. It studies the non-recoverable but photoreversible damage inactivating DNA replication and its relation to cell lethality. It attempts to explore the nature of the mutation induction process and determine its possible relation to the non-conservative DNA replication involved in excision-repair, to recombination processes and to repair mechanisms dependent on initiation of semi-conservative DNA replication after radiation exposure.